Here’s how I have my setup configured. I’ve got a total of 6 drives listed below:
2 X 500GB SSD’s
2 X 1TB SSD’s
2 X 6TB HDD’s
Each pair is RAID mirrored (RAID 1) to protect my data. So, both of the 500GB SSD’s are always exact copies of one another, 1TB SSD’s are always copies, etc.
I use the 500GB drives for all apps and personal files. The 1TB drives for sample libraries, vst’s, basically all audio stuff. And I use the 6TB drives for storage. As soon as I finish a project I move it onto the 6TB drives and delete it off the 500GB drives.
I’m concerned with my apps and libraries loading quickly which is why I have them on SSD’s. But, I don’t really care if the HDD’s take a while to load since they are just bulk storage. Mirroring all the drives is an insurance policy – they don’t need the other drive to work. As soon as one drive fails, I can use the working copy and copy all the data to a replacement drive. Then, you should be able to RAID the two drives (1 old, 1 new) since they should be exact copies. Luckily, I haven’t had a drive fail yet so I can’t tell you for certain if the “re-raiding” is as simple as I think it should be.
If you’re not as drive failure paranoid as I am, you can use one of the other RAID options listed here: http://www.pcworld.com/article/132877/RAID.html
Hope that helps and/or gives you some ideas for your setup!